Leishmania are typanosomatid protozoans that are transmitted by phlebotomine sand flies causing leishmaniasis. Leishmaniasis is a zoonosis and affects 12 million people in 88 countries, of which 72 are considered developing countries. It is estimated that 350 million people are at risk to infection by the different species of Leishmania. The annual incidence of new cases is about 2 million (1.5 million of cutaneous leishmaniasis, and 500,000 of visceral leishmaniasis). Like many other tropical diseases, the leishmaniases are related to economic development and man-made environmental changes, which increase exposure to the sandfly vector. The disease assumed importance in the United States as many Desert Storm veterans were exposed to sand flies and were afflicted with Leishmaniasis. Leishmania/HIV co-infection is emerging as an extremely serious, new disease and it is increasing in frequency. Treatment is based primarily on pentavalent antimony compounds although a new drug, miltefosine, shows promise in treating leishmaniasis. Unfortunately, the antimony compounds are toxic and resistance in various endemic regions is common. Miltefosine has only been recently approved in India and its effectiveness in the field and the development of resistance are unknown. We will exploit the availability of approximately 140,000 compounds whose structures and mammalian cytotoxicity are known and which have not been screened for anti-Leishmania activity. In the Phase I work, we will screen 15,000 compounds for anti-Leishmania activity and test compounds for efficacy first in vitro and then in vivo. We will accomplish this in Two Specific Aims: Aim One: Screen 15,000 compounds for in vitro activity against Leishmania. Active compounds will be screened against fungi and bacteria and only Leishmania specific compounds that have no mammalian cell toxicity will be tested for in vitro efficacy with cultures Leishmania-infected macrophages. Aim Two: Those compounds that are toxic for Leishmania in vitro will then be tested in a cutaneous murine model of L. major infection. Compounds that are found to have in vivo activity will be developed further in Phase II as well as an additional 125,000 compounds screened for activity. Ultimately, this work will lead to the identification of a lead compound for human clinical trials for the treatment of leishmaniasis.